Resumen: Interest in using clays in the diets of ruminants to improve health and performance is increasing. The microbial fermentation of 65:35 (HC) or 35:65 (HF) concentrate:forage feeds, alone or with zeolite (Z), bentonite (B), or sepiolite (S), was studied in an in vitro semicontinuous culture system. The medium pH was allowed to drop for the first 6 h and was gradually buffered thereafter. For the HC diet, the medium pH was higher with Z throughout incubation (p 0.05) were observed in dry matter disappearance, microbial mass, or volatile fatty acids. However, the inclusion of B in HC reduced the ammonia concentration at 6 and 12 h with respect to C (p < 0.05). The inclusion of zeolite as an additive in the diets of ruminants stabilizes the rumen environment during the first stages of fermentation in terms of pH and ammonia concentration, especially in high-concentrate diets. The buffering effect of bentonite and sepio-lite was lower, and both might reduce ruminal microbial fermentation, depending on the concentrate proportion. Idioma: Inglés DOI: 10.3390/ani12030345 Año: 2022 Publicado en: Animals 12, 3 (2022), 345 [11 pp.] ISSN: 2076-2615 Factor impacto JCR: 3.0 (2022) Categ. JCR: VETERINARY SCIENCES rank: 13 / 144 = 0.09 (2022) - Q1 - T1 Categ. JCR: AGRICULTURE, DAIRY & ANIMAL SCIENCE rank: 12 / 62 = 0.194 (2022) - Q1 - T1 Factor impacto CITESCORE: 4.2 - Veterinary (Q1) - Agricultural and Biological Sciences (Q2)